A technique for generating stereo sounds is disclosed in Japanese Publication No. 9397/1985 (hereafter, JP9397). An audio signal is output as an analog signal immediately, and then after a delay, is output again to produce stereo sounds. However, JP9397 cannot be applied to a digital sound source. Nor does JP9397 take into account movement of an object or a character displayed on a screen of a CRT display. As a result, sounds associated with the object are not synchronized with the movement of the object.
Japanese Patent Laid-Open No. 155879/1987 (hereafter JP 155879) discloses a sound generator for generating two-dimensional sounds in relation to an image display. JP155879 controls the sound volumes of left and right speakers in a two-dimensional manner, such as for an airplane, by gradually decreasing the sound volume as the airplane moves away and gradually increasing the sound volume as the airplane moves closer.
However, the sound volume is only gradually decreased or increased as the airplane moves which may not give a desired realistic effect. Even when the sounds are heard in stereo, three-dimensional sound effects are not obtained making JP155879 unsuitable for a three-dimensional image display. The problem is further complicated since three-dimensional sound effects should be synchronized with three-dimensional image movement. In addition, as the difference between the sound volumes of the right and left sounds increases, the user is easily fatigued. It has been experimentally shown that the use of headphones with differing left and right ear volumes for an extended time often gives the listener a headache.